Motor vehicle transmission systems transfer drive from a motor to wheels of the vehicle. Off-road vehicles, for example, often feature high and low range transmissions typically consisting of a first gearbox arranged to provide several gear ratios and a second gearbox arranged to provide two gear ranges. Combination of the first and second gearboxes provides a first and second set of gear ratios each being high or low range relative to the other. Control of such transmissions is typically provided by two selectors. A first selector may be, for example, a conventional gear lever arranged to select one of the several gear ratios in the first gearbox. In this way the first gear box is equivalent to a gearbox in a vehicle without high and low ranges. A second selector may be, for example, a switch to select one of the two ranges in the second gearbox. The switch might, for example, be labelled “high/low range”.
In operation, a driver of a vehicle having a high and low range transmission has to operate the two selectors to move between high and low ranges. This can be problematic when, for example the first gearbox is in a second ratio gear and the second gearbox is in a low range gear, and the driver wishes to change to the first gearbox being in a first ratio gear and the second gearbox being in a high range gear. To complete such a change while the vehicle is in motion, the driver must operate the two selectors to change gear, in any order, in each of the first and second gearboxes. An interim period between changing gear in each gearbox results in the combined transmission being either a first ratio gear in the first gearbox and a low range gear in the second gearbox, or a second ratio gear in the first gearbox and a high range gear in the second gearbox. In these interim scenarios, the combined ratio is lower than the original ratio or higher than the desired ratio respectively, and thus may cause the vehicle to jump. Accordingly, vehicles having high and low range transmissions are usually restricted to allowing change of the second gearbox gearing only when the vehicle is stationary. This is not desirable in scenarios where the driver wishes to move from low to high ranges smoothly, such as when driving from a field to a road, or when towing a trailer carrying a boat, for example, from a lake onto a road. In the latter example the driver has to stop the vehicle once the trailer is removed from the lake in order to change range to an appropriate range for road driving.